The emphasis in the past two or so decades on conserving energy has led to substantial improvements in the efficiency of more recently designed furnaces for controlling the air temperature in interior space. However, these high efficiency furnaces are more expensive, and sometimes this higher cost does not justify the energy conservation possible. Furthermore, the typical furnace is a relatively long-lived device. The result of these factors is that there is now a wide range of efficiencies in furnaces which are already installed or to be installed as new or replacement units.
There are two different types of heating systems using combustion of a fuel now in common use. The most common of these is the heated air system which uses a fan to force air heated by the furnace through ducts to the various rooms or areas of the structure. A less common type of heating system typically but not always found in older homes is the hydronic system which circulates water heated by the furnace through radiators located in the rooms or areas of the structure to be heated. Most of the discussion which follows will assume a heated air system, but the invention to be described below can be practiced in a hydronic system as well, although the operating parameters of the invention will be different. Within these two types of heating systems, there are further subtypes of which no further note be taken, other than to observe that the parameters used by the invention when installed in a heating system will again differ from one subtype to another.
As is almost too well known to requiring mentioning, furnace operation is controlled by a furnace control which sequences the various functions of the heating system so as to safely start, run., and stop the heating system. One or more thermostats installed within the structure whose interior space is to be heated, controls flow of current from a low voltage power supply such as a transformer or a DC converter to the furnace control. In the typical procedure, each time power is applied to the furnace control the heating system runs through its normal operating sequence to the run condition where heat is provided to the structure. When power is removed from the furnace control, the furnace then reenters its wait (standby) state.
Thermostats are typically of two types also, electronic and electromechanical. While electronic thermostats typically offer more features, they are also somewhat more expensive, and their more complicated user interface tends to confuse technically challenged individuals. Accordingly, there is still a substantial market for electromechanical thermostats. The invention pertains to electromechanical thermostats only.
In typical electromechanical thermostats, the angle relative to horizontal of a mercury switch is controlled by a bimetal coil. As is well known of course, the conductive state of the mercury switch is controlled by this angle. The inner end of the bimetal coil is mounted for rotation on an axle which can be placed at any of a variety of angular positions which defines the thermostat temperature setting. The mercury switch is mounted on the outer end of the bimetal coil, and as the temperature of the bimetal coil changes, the angle with respect to horizontal of the mercury switch also changes. Since the bimetal coil has very small mass compared to its surface area, the angle of the switch changes quite rapidly as temperature of the air surrounding the coil changes. When used to control a furnace, contacts within the mercury switch are bridged by the mercury globule within the mercury switch as the temperature of the bimetal coil falls. As the temperature of the bimetal coil rises in response to operation of the furnace, the change in the mercury switch angle causes the mercury globule to roll away from the contacts, and conduction between them ceases, shutting down the furnace.
The bimetal coil and switch of a typical thermostat have a switch differential of around 2.degree. F., meaning that an increase of 2.degree. F. in the air temperature adjacent to the bimetal coil after the switch closes is sufficient to cause the switch to open, and a decrease in this air temperature of 2.degree. F. after the switch opens is sufficient to cause the switch to close. The switch differential is related to the so-called room differential, which is the swing in room temperature necessary to change the switch conduction state. One will realize that the room differential is dependent on a number of factors in addition to the switch differential. Among these is the amount of air circulation in the room and the size and location of vents in the thermostat housing itself. One should also realize that the air temperature within a room or area is not uniform. Furthermore, comfort of the occupants depends on a number of factors besides air temperature within the structure. Among these factors are humidity, wall temperature, window area and window treatments, outside air temperature.
One factor in operation of furnaces which is controlled at least partly by the thermostat is the cycle time, or the time between successive startups of the furnace. Cycle time is usually measured not in the actual time between successive startups but instead in terms of the number of startups or cycles per hour, abbreviated cph. Thus a cycle time of 20 minutes is the equivalent of 3 cph. It is preferred to have a lower cycle rate, typically 3 cph, for high efficiency furnaces for a variety of reasons. Chief among these is the fact that the combustion gasses ejected from a high efficiency furnace are cooled to a level which is very near to the condensing temperature of the water vapor in the combustion gasses. This causes moisture to condense in the chimney duct and flue during each startup of the furnace. If the cph value is high, the moisture can accumulate because the flue does not get a chance to thoroughly heat and evaporate any condensed moisture. Since these chimney ducts and flues at least partly comprise galvanized steel, accumulated moisture eventually causes rusting and even perforation of the duct. Perforation of the duct in particular is a serious situation since it may allow release of toxic combustion products within living spaces. Less efficient furnaces release combustion gasses at a higher temperature which tends to thoroughly heat and dry out the chimney duct even with a high cycle rate. It is therefore possible to run less efficient furnaces at higher cycle rates without harm to the flues and ducts. A common cycle rate for furnaces having conventional efficiencies is 5 cph. Other things being equal (which they not usually are), it is preferable to run at a higher cycle rate because the room differential is smaller for higher cycle rates. However, when using a high efficiency furnace, one can compensate for the larger room differential resulting from a lower cycle rate by simply increasing the temperature setting slightly for the thermostat.
Experience has shown that a thermostat comprising only the bimetal coil and switch tends to produce quite a large room differential because of the conduction and convection heat transfer delays between the furnace and the room air and between the room air and the air within the thermostat housing. That is, once the thermostat switch closes, several minutes at least elapse before the room air temperature starts to rise. And after the room air temperature has risen to the comfort level, the temperature within the thermostat housing will lag behind this temperature by several minutes. When the switch differential range has been traversed by the air temperature adjacent to the bimetal coil, the switch opens and the furnace shuts down. However, the room air temperature will continue to rise for some period of time as the heat of the furnace continues to enter the room through a variety of mechanisms.
This undesirably large room differential created by a bimetal coil and switch alone is corrected by the use of a so-called anticipator resistor. An anticipator resistor for heating control is connected within the thermostat circuit so as to conduct current whenever power is supplied to the furnace control. The anticipator resistor is placed in physical proximity to the bimetal coil within the thermostat housing. When power is supplied to the furnace control, the current also flowing through the anticipator resistor generates heat which raises the temperature in the vicinity of the bimetal coil, typically to a level slightly higher than that of the surrounding room air. This causes the thermostat switch to shut down the furnace before the temperature of the room air exceeds the switch differential's upper limit defined by the thermostats present temperature setting. In this way, the room differential is greatly reduced.
A further aspect of room differential relates to cycle rate. A very large room differential (or switch differential for that matter) implies a relatively low cycle rate. This stands to reason because longer warm-up and cooldown times for the heated space are implied by a larger room differential. In general, the size of the anticipator resistor is used to set the cycle rate compatible with the characteristics of the furnace.
There are two types of anticipator connections. The so-called voltage anticipator is connected in parallel with the furnace control. The value of a voltage anticipator typically runs a few thousand ohms for 24 VAC control voltage. Such an anticipator resistor can be shipped with a fixed value which will provide the desired cycle rate for any type of furnace control. The disadvantage with this type of connection is that three wires are needed to connect the thermostat into the circuit. If the thermostat is to be installed in a new structure or a structure undergoing remodeling, it is easy to run a third wire.
Where it is not easy to run a third wire, then it is usually preferred to use a conventional anticipator connection, where the anticipator resistor is placed in series with the furnace control. The disadvantage of this arrangement is that the resistor value required for the desired cycle rate varies with the current which the furnace control draws. The procedure which has been adopted is to provide as the anticipator an adjustable resistor having a scale which is calibrated with a number of different current values. At the time the thermostat is installed (or the furnace control is changed), this resistor is set to the value corresponding to the current drawn by the furnace control. By selecting the proper anticipator resistor value, accurate temperature control of the space and the proper cycle rate both result.
The preferred adjustable anticipator resistor is formed by winding small resistive wire around a form which somewhat resembles a fish's body, and hence anticipator resistors formed in this fashion are called fishtails. A small slider can be shifted to make contact with the fishtail at any turn, and in this way the resistance of the fishtail can be easily set to the appropriate value. In general the manufacturer will provide a scale which relates the slider position to the current drawn by the furnace control. In one design now commercially available from Honeywell, Inc., Minneapolis, Minn. and known as the T84 1D thermostat, letters are applied to the fishtail to designate the position of the slider, and the instructions enclosed with the thermostat relate the various letter values to the furnace control current.
In the past, it has been necessary to provide different thermostats for use with heating plants having furnaces with different preferred cycle rates. This adds the expense of stocking different thermostat models designed to provide different cycle rates.